Amorphous indium zinc oxide (IZO) thin film structures of varying amounts of Zn content were investigated using X-ray diffraction measurements and molecular dynamics (MD) simulations. The characteristic amorphous structure having high oxygen coordination number and edge-shared polyhedra were confirmed using both techniques. Detailed analysis of the structural model revealed that the oxygen close-packed structure was almost realized in the nanometer range. It was also found that the number of Zn ions occupying the tetrahedral site of the oxygen close-packed structure increased with increasing ZnO content although In ions occupied the octahedral site. We conclude that the amorphous structure stability of the indium zinc oxide thin films is enhanced by the existence of Zn ions in the tetrahedral site, which block In ions in the octahedral site ordering similar to that in an In2O3 crystal.